1. Field of the Invention
The present invention relates to a filter unit for filtering gas and having a plurality of bag filters hanging down from a suspension plate dividing a housing into a lower gas inlet side and an upper outlet side for filtered gas, and a plurality of cleaning nozzle devices arranged on the outlet side of the bag filters, in which the individual bag filter has at least an open end at the suspension plate, a tubular wall of a flexible filter material, and a lower end.
2. Description of Related Art
A filter unit of this kind is known from WO 98/00001 (Niro). Such bag filters are only rigidly fixed at one end at the suspension plate, and the flexible filter material is closed into an end bottom at the opposite, lower end. The bag filters typically have a length of at least several meters, and a reverse jet cleaning nozzle device ejects pulses of cleaning air down through the open upper end of the bag filter at regular intervals during the filtration process in order to clean deposited product off the outside of the filter bag. The pressure of the pulse of cleaning air needs to be relatively high in order for the pulse to penetrate down to the lower portion of the bag filter and subject the filter material in that area to an adequate pressure increase to knock loose the deposited product. The pressure is in particular required to be relatively high when the bag filter has a relatively long length.
In between periods of operation the filter unit throughout is cleaned, e.g. by a cleaning-in-place process (CIP) involving cleaning nozzles located at least in the clean gas chamber at the upper outlet side of the bag filters. The cleaning nozzles are supplied with a cleaning liquid that possibly includes a cleaning agent. It is also possible to supply the cleaning nozzles with gas pulses in combination with the supply of cleaning liquid. During cleaning the cleaning liquid is washed down into the bag filters together with any entrained particles or powder from the clean gas side.
The liquid flows through the bag filters, but particles or powders can accumulate at the bottom of the bag filters, resulting in a somewhat imperfect cleaning of the filter unit.
The pressures from the pulses ejected during filtration or in between periods of operation act on the flexible filter material and will during time cause mechanical wear which can result in local areas with too high a porosity. Cleaning agents used in the cleaning liquid can weaken the filter material and make it more vulnerable to machanical influences. Formation of small holes in the lower end portion of the bag filter may occur, which shortens life span of the bag filters.
An object of the present invention is to improve the cleanliness of a freshly cleaned filter unit and also the life span of the bag filters.
In view of this the filter unit according to the present invention is characterized in that the lower end of the individual bag filter is an end closure of an impermeable material and provided with a drain, that said tubular wall has a downwards open end fixed to the end closure, and that the end closures of the bag filters are interconnected.
The impermeable end closure is durable, non-filtering and has a drain through which cleaning liquid and any particles or other remains from the cleaning process can be flushed out. The cleaned and drained bag filters have no remains or deposits of particles or dust on their inside, and a CIP process results in a completely sanitary filter unit.
A further advantage of the present invention is a reduction of the drying time required from finished cleaning to initiation of normal filtering operation. The cleaned bag filters contain only small amounts of cleaning liquid due to the draining, and also due to the impermeable end closure material which cannot draw in liquid.
The pressure pulse, whether ejected during filtration or in between filtration processes, loads the filter material with hoop stresses, but the vertically directed tensile stresses, which in the prior art bag filters are caused by the pressure pulse hitting the end bottom, have been avoided or at least effectively diminished by making the tubular wall of flexible filter material with an open end. The pressure pulse hits the end closure and due to the interconnections between the end closures the downwards directed force from the pressure pulse is distributed to the neighbouring end closures or to the housing through a support structure. The result is a lower loading on the filter wall material for a given pulse pressure.
The interconnection of the end closures also lock their mutual positions in the horizontal direction, and this prevents the tendency to formation of small holes in the lower section of the bag filters. In the prior art filter units it is presumed that sideways movement of individual or a group of bag filters with respect to the remaining bag filters could happen due to the flow of incoming gas into the filter unit with the consequences that a bag filter would rub against or hit a neighbouring bag filter. Also, sideways movement of individual bag filters could sometimes be caused by the pressure pulses hitting the end bottom. The interconnections prevent such horizontal movements of the bag filters. The lower internal loading on the filter wall material and the avoidance of external local loading from contact with other bag filters are both factors leading to a longer life span of the bag filters.
An additional advantage of the present invention resides in the possibility for mounting the bag filters in a more close or dense configuration, and/or to use bag filters of longer length.
Preferably, the end closures of the bag filters are substantially stationary mounted in the filter housing. This provides a mechanically simple structure that facilitates cleaning of the space below the suspension plate. The stationary mounting is also an advantage when the lower ends of the tubular walls of flexible material have to be fixed to the associated end closures. As an alternative to the stationary mounting, the end closures can be mounted with a possibiliy for limited adjustment of their position in the vertical direction, such as by interposing a hydraulic cylinder between the interconnected end closures and the filter housing. In this manner the end closures can be precisely positioned in accordance to the current actual length of the tubular walls of the bag filters, because the upper ends of the bag filters remain stationary fixed at the suspension plate. Another possibility for adapting to the current length of the bag filters is to use stationary end closures and spring loaded or flexible tubular wall end portions interposed between the end closures and the lower end portions of the bag filters.
In an embodiment that is well suited for large filter units having many bag filters, each end closure is fixed onto a drain pipe, and the drain pipes with end closures are arranged in at least one row on a transversely extending, rigid collecting pipe connected to and supporting the drain pipes with end closures in said row. The drains in this row can be commonly controlled by a valve in the collecting pipe. For sanitary reasons it is an advantage to use a collecting pipe, because the drain exit from the individual end closure is shielded by the collecting pipe from the dusty gas inlet chamber.
For further improvement of sanitary conditions and facilitated cleaning the individual collecting pipe preferably has a diamond-like cross-sectional shape with the longer axis extending upwards. The collecting pipe has an upwards facing surface which is exposed to product settling. By making the collecting pipe with an oval or more regular diamond-like shape, the upwards facing, almost horizontal portion of the pipe becomes smaller and the side faces obtains a steeper slope which counteracts product deposits on top of the pipe.
Preferably, the sanitary conditions are improved by letting the individual collecting pipe extend out of the housing of the filter unit so that the pipe has no free opening to the gas inlet chamber at the lower side of the suspension plate. With this design the collecting pipe(s) and the drains in the end closures are only open to the clean gas side and have only to carry cleaning liquid and possible product remains washed away from the inside of the bag filters. A further advantage is that valves, connections etc. associated with the collecting pipes can be located outside the filter housing which reduces the number and size of upwards facing surfaces located inside the filter housing.
A further sanitary improvement can be obtained by making both ends of the individual collecting pipe extend out of the housing, and that the non-draining end of the collecting pipe is connected via a valve to a source of cleaning liquid or drying gas. This allows the collecting pipes to be cleaned by flushing with cleaning liquid and thus become independent of the amount of cleaning liquid flowing down through the bag filters.
In an embodiment the individual bag filter is designed so that a filter basket providing radial support to the tubular wall of the bag filter in a well-known manner, is supported by the end closure. By supporting the filter basket the end closure also reduces mutual movements between the basket and the end closure and the tubular wall fixed thereto. In addition the suspension plate is relieved from some of its load.
In an embodiment that is preferred due to its ability for being relatively quickly and completely cleaned during a CIP-process the flexible filter material is, in between end portions, a non-folded cloth or felt, and in each end portion the filter material is provided with a ringshaped stiffener. At the top and the bottom ends the stiffener is during mounting of the tubular wall of the filter bag snap locked into engagement with the suspension plate and the end closure, respectively.
It is an advantage to design the top and bottom ends of the tubular wall of the bag filter so that they are identical. When mounting such a bag filter in the filter unit it does not matter which end is turned upwards and so mounting is facilitated.
In a further development the cloth is of a synthetic material and includes a coating of PTFE. The PTFE coating provides the filter material with a diminutive mesh size, but due to the drainage at the end closure it is nevertheless with the present invention possible to obtain fully sanitary conditions during cleaning of the bag filter. This allows for production of very fine powders by use of a bag filter having an economical design.
In a very simple and durable embodiment the end closure is a metal cup with a central drain hole and an inwards protruding upper rim area. A snap lock ring in the lower end of the tubular wall can easily be mounted behind the inwards protruding rim.
The decoupling of the pulse pressures from the longitudinal tensile stress loading of the filter material allows an increase of the pulse pressures without overloading the filtering material. According to the present invention it is possible to make the individual bag filter with a length of at least 8.5 m. Sufficient cleaning of the long bag filter can be obtained by raising the pulse pressure.